Swiftでarray.permutation ref: http://qiita.com/takeo-asai/items/64fa975991e49fb58f0e

file0.swift

let a = [1,2,3]
a.permutaion(2)
// => [[1, 2], [2, 1], [1, 3], [3, 1], [2, 3], [3, 2]]

file1.swift

let a = [1,2,3]
a.combination(2)
// => [[1, 2], [1, 3], [2, 3]]
a.repeatedPermutation(2)
// => [[1, 1], [1, 2], [1, 3], [2, 1], [2, 2], [2, 3], [3, 1], [3, 2], [3, 3]]

file2.swift

let a = [1, 2, 3]
var p = PermutationGenerator(elements: a, indices: [1, 2, 0])
p.next()
// => Optional(2), =a[1]
p.next()
// => Optional(3), =a[2]
p.next()
// => Optional(1), =a[0]
p.next()
// => nil